var mergeTwoLists = function(list1, list2) {
    if (!list1 || !list2) return list1 ? list1 : list2;
    if (list1.val < list2.val) {
        list1.next = mergeTwoLists(list1.next, list2);
        return list1;
    } else {
        list2.next = mergeTwoLists(list1, list2.next);
        return list2;
    }

};
l1 = [1, 4, 7, 9];
l2 = [1, 3, 4, 6, 8];
mergeTwoLists(l1, l2);